Sang-Ha Baik

Oxidative lipid modification of nicastrin enhances amyloidogenic γ-secretase activity in Alzheimer's disease.

The cause of elevated level of amyloid β‐peptide (Aβ42) in common late‐onset sporadic [Alzheimer’s disease (AD)] has not been established. Here, we show that the membrane lipid peroxidation product 4‐hydroxynonenal (HNE) is associated with amyloid and neurodegenerative pathologies in AD and that it enhances γ‐secretase activity and Aβ42 production in neurons. The γ‐secretase substrate receptor, nicastrin, was found to be modified by HNE in cultured neurons and in brain specimens from patients with AD, in which HNE–nicastrin levels were found to be correlated with increased γ‐secretase activity and Aβ plaque burden. Furthermore, HNE modification of nicastrin enhanced its binding to the γ‐secretase substrate, amyloid precursor protein (APP) C99. In addition, the stimulation of γ‐secretase activity and Aβ42 production by HNE were blocked by an HNE‐scavenging histidine analog in a 3xTgAD mouse model of AD. These findings suggest a specific molecular mechanism by which oxidative stress increases Aβ42 production in AD and identify HNE as a novel therapeutic target upstream of the γ‐secretase cleavage of APP.

Inhibition of Notch signalling ameliorates experimental inflammatory arthritis

Objective To test the hypothesis that Notch signalling plays a role in the pathogenesis of rheumatoid arthritis (RA) and to determine whether pharmacological inhibition of Notch signalling with γ-secretase inhibitors can ameliorate the RA disease process in an animal model. Methods Collagen-induced arthritis was induced in C57BL/6 or Notch antisense transgenic mice by immunisation with chicken type II collagen (CII). C57BL/6 mice were administered with different doses of inhibitors of γ-secretase, an enzyme required for Notch activation, at disease onset or after onset of symptoms. Severity of arthritis was monitored by clinical and histological scores, and in vivo non-invasive near-infrared fluorescence (NIRF) images. Micro-CT was used to confirm joint destruction. The levels of CII antibodies and cytokines in serum were determined by ELISA and bead-based cytokine assay. The expression levels of cytokines were studied by quantitative PCR in rheumatoid synovial fibroblasts. Results The data show that Notch signalling stimulates synoviocytes and accelerates their production of proinflammatory cytokines and immune responses involving the upregulation of IgG1 and IgG2a. Pharmacological inhibition of γ-secretase and antisense-mediated knockdown of Notch attenuates the severity of inflammatory arthritis, including arthritis indices, paw thickness, tissue damage and neutrophil infiltration, and reduces the levels of active NF-κB, ICAM-1, proinflammatory cytokines and matrix metalloproteinase-3 activity in the mouse model of RA. Conclusions These results suggest that Notch is involved in the pathogenesis of RA and that inhibition of Notch signalling is a novel approach for treating RA.

Pin1 promotes neuronal death in stroke by stabilizing Notch intracellular domain

Stroke is a leading cause of mortality and disability. The peptidyl‐prolyl cis/trans isomerase Pin1 regulates factors involved in cell growth. Recent evidence has shown that Pin1 plays a major role in apoptosis. However, the role of Pin1 in ischemic stroke remains to be investigated.

Selenium attenuates Aβ production and Aβ-induced neuronal death

The objective of the present study was to examine the role of selenium in the metabolism of A beta and in A beta-induced neuronal death. Selenium treatment significantly reduced A beta 40, A beta 42, and sAPP beta production by reducing A beta producing beta-secretase and gamma-secretase activities. The lipid peroxidation product 4-Hydroxynonenal (HNE)-induced transcription of beta-secretase (BACE1) was blocked by selenium. Finally, our data show that selenium protects against HNE and A beta-mediated toxicity in primary cultured neurons. The present study suggests that selenium may be able to salvage the neuronal degeneration of Alzheimers disease, thereby limiting beta-amyloid production and neuronal death.

Secretases as therapeutic targets for Alzheimer's disease.

Accumulation of amyloid-β (Aβ) is widely accepted as the key instigator of Alzheimers disease (AD). The proposed mechanism is that accumulation of Aβ results in inflammatory responses, oxidative damages, neurofibrillary tangles and, subsequently, neuronal/synaptic dysfunction and neuronal loss. Given the critical role of Aβ in the disease process, the proteases that produce this peptide are obvious targets. The goal would be to develop drugs that can inhibit the activity of these targets. Protease inhibitors have proved very effective for treating other disorders such as AIDS and hypertension. Mutations in APP (amyloid-β precursor protein), which flanks the Aβ sequence, cause early-onset familial AD, and evidence has pointed to the APP-to-Aβ conversion as a possible therapeutic target. Therapies aimed at modifying Aβ-related processes aim higher up the cascade and are therefore more likely to be able to alter the progression of the disease. However, it is not yet fully known whether the increases in Aβ levels are merely a result of earlier events that were already causing the disease.

Inhibition of Drp1 Ameliorates Synaptic Depression, Aβ Deposition, and Cognitive Impairment in an Alzheimer's Disease Model

Excessive mitochondrial fission is a prominent early event and contributes to mitochondrial dysfunction, synaptic failure, and neuronal cell death in the progression of Alzheimers disease (AD). However, it remains to be determined whether inhibition of excessive mitochondrial fission is beneficial in mammal models of AD. To determine whether dynamin-related protein 1 (Drp1), a key regulator of mitochondrial fragmentation, can be a disease-modifying therapeutic target for AD, we examined the effects of Drp1 inhibitor on mitochondrial and synaptic dysfunctions induced by oligomeric amyloid-β (Aβ) in neurons and neuropathology and cognitive functions in Aβ precursor protein/presenilin 1 double-transgenic AD mice. Inhibition of Drp1 alleviates mitochondrial fragmentation, loss of mitochondrial membrane potential, reactive oxygen species production, ATP reduction, and synaptic depression in Aβ-treated neurons. Furthermore, Drp1 inhibition significantly improves learning and memory and prevents mitochondrial fragmentation, lipid peroxidation, BACE1 expression, and Aβ deposition in the brain in the AD model. These results provide evidence that Drp1 plays an important role in Aβ-mediated and AD-related neuropathology and in cognitive decline in an AD animal model. Therefore, inhibiting excessive Drp1-mediated mitochondrial fission may be an efficient therapeutic avenue for AD. SIGNIFICANCE STATEMENT Mitochondrial fission relies on the evolutionary conserved dynamin-related protein 1 (Drp1). Drp1 activity and mitochondria fragmentation are significantly elevated in the brains of sporadic Alzheimers disease (AD) cases. In the present study, we first demonstrated that the inhibition of Drp1 restored amyloid-β (Aβ)-mediated mitochondrial dysfunctions and synaptic depression in neurons and significantly reduced lipid peroxidation, BACE1 expression, and Aβ deposition in the brain of AD mice. As a result, memory deficits in AD mice were rescued by Drp1 inhibition. These results suggest that neuropathology and combined cognitive decline can be attributed to hyperactivation of Drp1 in the pathogenesis of AD. Therefore, inhibitors of excessive mitochondrial fission, such as Drp1 inhibitors, may be a new strategy for AD.

Evidence for a detrimental role of TLR8 in ischemic stroke

Toll-like receptors (TLRs) are transmembrane pattern-recognition receptors that initiate signals in response to diverse pathogen-associated molecular patterns. Several groups have recently reported a role for TLR2 and TLR4 in ischemic stroke-induced brain injury. However, relatively little is known about the role of TLR8 in ischemic stroke. Here we provide the first evidence that TLR8 activation plays a detrimental role in stroke outcome by promoting neuronal apoptosis and T cell-mediated post-stroke inflammation. TLR8 is expressed in cerebral cortical neurons, where its levels and downstream signaling via JNK are increased in response to oxygen glucose deprivation (OGD). Treatment with a TLR8 agonist activated pro-apoptotic JNK and increased neuronal cell death during OGD. Furthermore, selective knockdown of TLR8 using siRNA protected SH-SY5Y cells following OGD, and TLR8 agonist administration in vivo increased mortality, neurological deficit and T cell infiltration following stroke. Taken together, our findings indicate a detrimental role for neuronal TLR8 signaling in the triggering of post-stroke inflammation and neuronal death.

Effects of chronic alcohol consumption on expression levels of APP and Aβ-producing enzymes.

Chronic alcohol consumption contributes to numerous diseases, including cancers, cardiovascular diseases, and liver cirrhosis. Epidemiological studies have shown that excessive alcohol consumption is a risk factor for dementia. Along this line, Alzheimers disease (AD) is the most common form of dementia and is caused by the accumulation of amyloid-β (Aβ plaques in neurons. In this study, we hypothesized that chronic ethanol consumption is associated with pathological processing of APP in AD. To investigate the relationship between chronic alcohol consumption and Aβ production, brain samples from rats fed an alcohol liquid diet for 5 weeks were analyzed. We show that the expression levels of APP, BACE1, and immature nicastrin were increased in the cerebellum, hippocampus, and striatum of the alcohol-fed group compared to the control group. Total nicastrin and PS1 levels were induced in the hippocampus of alcohol-fed rats. These data suggest that the altered expression of APP and Aβ-producing enzymes possibly contributes to the chronic alcohol consumption-mediated pathogenesis of AD.

Contribution of γ-secretase to calcium-mediated cell death

Presenilins are the catalytic subunit of the large gamma-secretase complex, that promotes intramembranous proteolysis of the beta-amyloid precursor protein (APP), resulting in the production of beta-amyloid (A beta). Mutant presenilin causes early-onset familial Alzheimers disease (FAD), is related to abnormal Ca(2+) signaling, and render cells vulnerable to cell death. In the present study, we demonstrated that Ca(2+)-mediated cell death is functionally associated with gamma-secretase activity. We found that gamma-secretase activity was elevated during Ca(2+)-mediated cell death. Using selective gamma-secretase inhibitors, we examined the role of gamma-secretase in cell death triggered by increased intracellular Ca(2+). Indeed, treatment with the selective gamma-secretase inhibitors, compound E, DAPT, or L-685.458 significantly decreased Ca(2+)-triggered cell death with that of the controls, but did not affect staurosporin or tunicamycin-mediated cell death. These results implicate the role of gamma-secretase activity in Ca(2+)-mediated cell death.

Calsenilin Contributes to Neuronal Cell Death in Ischemic Stroke

Calsenilin is a calcium sensor protein that interacts with presenilin and increases calcium‐triggered neuronal apoptosis, and γ‐secretase activity. Notch is a cell surface receptor that regulates cell‐fate decisions and synaptic plasticity in brain. The aim of the present study was to characterize the role of calsenilin as a regulator of the γ‐secretase cleavage of Notch in ischemic stroke. Here, we determined the modulation of expression level and cellular distribution of calsenilin in neurons subjected to ischemic‐like conditions. The levels of calsenilin and presenilin were increased in primary neurons after oxygen and glucose deprivation. Furthermore, calsenilin was found to enhance the γ‐secretase cleavage of Notch and to contribute to cell death under ischemia‐like conditions. The inhibition of γ‐secretase activity and a presenilin deficiency were both found to protect against calsenilin‐mediated ischemic neuronal death. The expression of calsenilin was found to be increased in brain following experimental ischemic stroke. These findings establish a specific molecular mechanism by which the induction of calsenilin enhances Notch activation in ischemic stroke, and identify calsenilin as an upstream of the γ‐secretase cleavage of Notch.